A North American red squirrel in search of spruce cones to cache away for the rainy (or rather: snowy) day. Photo by: Anni Hämäläinen.

For a squirrel baby boomer, timing is everything
It is mid-winter and the forest rattles with frenzied squirrel activity – they chatter and chase one another in ongoing negotiations about fleeting romance. This year, the stakes are high and the squirrels know it. Using cues unknown to us, they have correctly anticipated times of plenty in the coming fall, as the spruce trees in their forest are about to fill their branches – and the squirrels’ larders – with more cones than all the squirrels in the forest can eat. This kind of an opportunity comes once in a squirrel’s lifetime, as spruce “mast” seeding only happens every few years at an unpredictable frequency. When it happens, however, there is enough food around to support many more squirrels than at times of low cone production, and any babies born on the eve of such abundance will have a much higher chance of surviving the harsh winter ahead, relying on a pantry full of cones.

Evolutionary processes have ensured that the parents take full advantage of this prospect, because producing more offspring in such years can significantly increase the parents’ reproductive success and pass on those genes to future generations. Researchers working on squirrel populations of the Kluane Red Squirrel Project in the Yukon have previously discovered that squirrel females can produce multiple litters in the breeding season preceding a mast year, and “teen moms” are more common in those years, as yearling females are more likely to breed in mast years. For our latest study (Scientific Reports in August 2017), we analyzed the detailed breeding histories of female squirrels in those populations since 1986, and estimated how accurately squirrels match their reproductive efforts to the mast years. We then asked exactly how important it is for a squirrel’s fitness that their timing is right. To answer this question, we compared the total numbers of surviving offspring produced by those females that maximized their breeding efforts in anticipation of a mast year, relative to those who failed to do so.

To test the importance of the matching of mast years with high-intensity reproduction, we examined the annual number of babies a female produced over their lifetime. The typical squirrel lifespan is approximately 3 years, but some squirrels survive to 8 years of age. They can start reproducing in the year after they are born, or delay their first litter to 2 years of age or older. What happens in between varies significantly among individuals: some never succeed in producing a litter, while others breed in every year of their life, producing between one and fifteen pups in a given breeding season. Focusing on those females that successfully gave birth at least once, we determined the age at which they produced their highest number of pups, or their reproductive “peak”.

We then assessed whether females were able to match their reproductive peak to a mast year or not. This showed us that females were much more likely to achieve their personal high score in terms of pup production in a mast year if they were lucky enough to live through one – and for good reason. As we marked all pups individually soon after birth, we were able to then check which babies survived their first winter and thus reach maturity. This led us to discovered that when females matched their reproductive peak to a mast year, more of their pups survived to maturity. These females that maximized their breeding efforts in a mast year were less likely to survive long past that year, but her legacy would endure thanks to her genes that were passed along to the next generation of squirrels.

Most animals live in unstable habitats, in which breeding can be a gamble. Producing and raising offspring takes a lot of time and energy, and often reduces the lifespan of the parent when those resources are limited. An ability to interpret cues from the environment that allow the parent to anticipate opportunities available for their prospective offspring may change their reproductive “decisions” so that they forego breeding in one year, or give it everything they have in another year. In terms of evolution, this makes perfect sense: those individuals that solve this equation in the best way will contribute more to future generations, in a prime example of natural selection.

The average squirrel, like many other mammals, produces their highest number of offspring sometime in prime adulthood, with an increase in pup production in early life followed by declining performance at old age. There is vast variation in this age profile among individuals. It turns out that the timing as well as intensity of peak reproduction matters: females that can match their reproductive peak with favorable environmental conditions will produce more surviving offspring, and females that are able to produce more pups at their peak also contribute more squirrels in total to the next generation.

This youngster is receiving tags so that we can follow him through life. At this age, he is still being looked after by his mother, but in a few months’ time, he will have to leave home to find a vacant lot in the neighborhood and fill a pantry with cones that will get him through the winter. Photo by: Anni Hämäläinen.